1. Field of the Invention
The present invention relates generally to a method and apparatus for more accurately measuring the temperature of a surface using infrared measurement techniques and, more particularly, to such a method and apparatus which utilises a laser sighting device which is adapted to project at least a circumscribing laser sighting beam or beams for more clearly defining the periphery of the energy zone from which the temperature is measured. Generally speaking, this has been accomplished by directing the laser beam about the periphery of the energy zone; by the use of three or more stationary laser beams which are focused on the periphery of the energy zone; or by the use of a controlled single laser beam directed towards three or more predetermined locations on the periphery of the energy zone. In an alternative embodiment, a single laser beam may be rotated around the periphery of the energy zone using, for example, slip rings. In another embodiment, the single rotating laser may be pulsed on and off in a synchronised manner in order to produce a series of intermittent lines outlining the energy zone, thus increasing the efficiency of the laser by concentrating its total wattage in a smaller area, causing a brighter beam. Further, the circumscribing beam or beams may be used in conjunction with an additional beam directed at and defining a central spot, or larger central area, of the energy zone.
In yet another method and embodiment, at least one laser beam is subdivided by passing it through a diffraction grating, for example, into a plurality of three or more subdivision beams which can form a pattern of illuminated spot areas on a target whose energy zone is to be investigated with a radiometer. Herein `a plurality` means three or more, e.g. six or twelve.
2. Description of the Prior Art
Remote infrared temperature measuring devices (commonly referred to as infrared pyrometers or radiometers) have been used for many years to measure the temperature of a surface from a remote location. Their principle of operation is well known. All surfaces at a temperature above absolute zero emit heat in the form of radiated energy. This radiated energy is created by molecular motion which produces electromagnetic waves. Thus, some of the energy in the material is radiated in straight lines away from the surface of the material. Many infrared radiometers use optical reflection and/or refraction principles to capture the radiated energy from a given surface. The infrared radiation is focused upon a detector, analysed and, using well known techniques, the surface energy is collected, processed and the temperature is calculated and displayed on an appropriate display.
Examples of such infrared radiometers are illustrated at pages J-1 through J-42 of the Omega Engineering Handbook, Volume 2B. See, also, U.S. Pat. No. 4,417,822 which issued to Alexander Stein et al. On Nov. 29, 1983 for a Laser Radiometer; U.S. Pat. No. 4,527,896 which issued to Keikhosrow Irani et al. on Jul. 9, 1985 for an Infrared Transducer-Transmitter for Non-Contact Temperature Measurement; and U.S. Pat. No. 5,169,235 which issued to Hitoshi Tominaga et al. for Radiation Type Thermometer on Dec. 8, 1992. Also see Baker, Ryder and Baker, Volume II, Temperature Measurement in Engineering, Omega Press, 1975, Chapters 4 and 5.
When using such radiometers to measure surface temperature, the instrument is aimed at a target or `spot` within the energy zone on the surface on which the measurement is to be taken. The radiometer receives the emitted radiation through the optical system and is focused upon an infrared sensitive detector which generates a signal which is internally processed and converted into a temperature reading which is displayed.
The precise location of the energy zone on the surface as well as its size are extremely important to insure accuracy and reliability of the resultant measurement. It will be readily appreciated that the field of view of the optical systems of such radiometers is such that the diameter of the energy zone increases directly with the distance to the target. The typical energy zone of such radiometers is defined as where 90% of the energy focused upon the detector is found. Heretofore, there have been no means of accurately determining the perimeter of the actual energy zone unless it is approximated by the use of a `distance to target table` or by actual physical measurement.
Target size and distance are critical to the accuracy of most infrared thermometers. Every infrared instrument has a field of view (FOV), an angle of vision in which it will average all the temperatures which it sees. Field of view is described either by its angle or by a distance to size ratio (D:S). If the D:S=20:1, and if the distance to the object divided by the diameter of the object is exactly 20, then the object exactly fills the instruments field of view. A D:S ratio of 60:1 equals a field of view of 1 degree.
Since most infrared thermometers have fixed-focus optics, the minimum measurement spot occurs at the specified focal distance. Typically, if an instrument has fixed-focus optics with a 120:1 D:S ratio and a focal length of 60" the minimum spot (resolution) the instrument can achieve is 60 divided by 120, or 0.5" at a distance of 60" from the instrument. This is significant when the size of the object is close to the minimum spot the instrument can measure.
Most general-purpose infrared thermometers use a focal distance of between 20" and 60" (50 and 150 cm); special close-focus instruments use a 0.5" to 12" focal distance. See page Z54 and Z55, volume 28, The Omega Engineering Handbook, Vol. 28. In order to render such devices more accurate, laser beam sighting devices have been used to target the precise center of the energy zone. See, for example, pages C1-10 through C1-12 of The Omega Temperature Handbook, Vol. 27. Various sighting devices such as scopes with cross hairs have also been used to identify the center of the energy zone to be measured. See, for example, Pages C1-10 through C1-21 of The Omega Temperature Handbook, Vol. 27.
The use of a laser to pinpoint only the center of the energy zone does not, however, provide the user with an accurate definition of the actual energy zone from which the measurement is being taken. This inability frequently results in inaccurate readings. For example, in cases where the area from which radiation emits is smaller than the target diameter limitation (too far from or too small a target), inaccurate readings will occur.
One method used to determine the distance to the target is to employ an infrared distance detector or a Doppler effect distance detector or a split image detector similar to that used in photography. However, the exact size of the energy zone must still be known if one is to have any degree of certainty as to the actual area of the surface being measured. This is particularly true if the energy zone is too small or the surface which the energy zone encompasses is irregular in shape. In the case where the surface does not fill the entire energy zone area, the readings will be low and, thus, in error.
Similarly, if the surface is irregularly shaped, the readings will also be in error since part of the object would be missing from the actual energy zone being measured.
Thus, the use of a single laser beam only to the apparent center of the energy zone does not insure complete accuracy since the user of the radiometer does not know specifically the boundaries of the energy zone being measured.
As will be appreciated, none of the prior art recognises this inherent problem or offers a solution to the problems created thereby.
Proposals have been made in the prior art for indicating an energy zone area of a target surface by means visible to the eye at the target.
A first kind of such indication utilises multi-spectral light, as evidenced for example in the Japanese Publication No. S57-22521 which teaches the use of an incandescent light source to outline an energy zone at the target. Japanese Publication No. 62-12848 suggests a similar use of multi-spectral light to outline an energy zone at the target. Reference is made to Japanese case JP 63-145928.
Further, U.S. Pat. No. 4,494,881 EVEREST also suggests using a multi-spectral light source together with a beam splitter arrangement which permits the infra-red received beam and the multi-spectral light to utilise the same optical arrangement. EVEREST teaches the use of a visible light source such as an incandescent lamp or strobe light which is projected against the target surface, the temperature if which is to be measured. This adds additional energy to the same energy zone where the temperature measurement is to be taken, and this destroys accuracy. When EVEREST uses a beam splitter, the incandescent light beam causes the beam splitter to act as a radiator of infrared energy. When EVEREST uses a Fresnel lens, the light tends to elevate the temperature of the Fresnel lens, which in turn reflects back to the infra-red detector.
This manner of indication, utilising incoherent multi-spectral light, has the disadvantage amongst others that the multi-spectral light itself has a heat factor which can cause incorrect reading by the energy detecting means of the apparatus.
A laser is Light Amplification by Stimulated Emission of Radiation. This device was invented in 1960 to produce an intense light beam with a high degree of coherence. Atoms in the material emit in phase. Laser light is used in holography. A light beam is coherent when all component waves have the same phase. A laser emits coherent light, but ordinary electric incandescent light is incoherent in which atoms vibrate independently.
It is not possible simply to substitute a laser for an incandescent light source, because the incandescent beam is incoherent in nature, so that when projected parallel and in close proximity to the boundaries of the invisible infra-red zone, incandescent light inside the infra-red zone is reflected as heat energy. Moving the incandescent beam well away from the infra-red zone clearly does not permit accurate delineation of the target zone.
A second kind of energy zone indicator utilises coherent- laser light, as evidenced for example in U.S. Pat. No. 4,315,150 of DERRINGER, which is directed to a targeted infrared thermometer in which a laser is provided to identify the focal point, i.e. the center, of the energy zone, but there is nothing in DERRINGER to suggest causing more than two laser beams to outline the energy zone.
U.S. Pat. No. 5,085,525 BARTOSIAK ET AL teaches use of a laser beam to provide a continuous or interrupted line across a target zone to be investigated, but there is no suggestion to outline a target zone, nor to indicate a central point or central area of the target zone.
German patent publications of interest include:
DE-38 03 464; PA1 DE-36 07 679 to a laser sighting device. PA1 DE-32 13 955; to a beam splitter and to dual laser beams to indicate position and diameter of the energy zone.
All of the above noted prior art is hereby incorporated into this case by reference thereto.